The present invention relates to a multi-piece solid golf ball. More particularly, it relates to a multi-piece solid golf ball having excellent durability, excellent rebound characteristics and good shot feel at the time of hitting.
In the history of golf balls, a thread wound golf ball has been firstly developed. The thread wound golf ball is obtained by winding thread rubber in a stretched state on a solid or liquid center to form a thread wound core and covering it with a cover of balata, etc. having a thickness of 1 to 2 mm.
A two-piece solid golf ball has been subsequently developed, which is composed of a core formed from integrally molded rubber material and a thermoplastic resin cover (e.g. ionomer resin cover) formed on the core. The two-piece solid golf ball is easily produced because of simple structure, and has excellent rebound characteristics and excellent durability. Therefore, the two-piece solid golf ball is generally approved or employed by many golfers, mainly amateur golfers.
However, the two-piece solid golf ball exhibits harder and poorer shot feel at the time of hitting than the thread wound golf ball. It is difficult to put spin on the two-piece solid golf balls, resulting in poor controllability with approach shots. The difficulty of putting spin on the ball comes from the structural features of the solid golf ball that a ball velocity is high, when hitting, and a contact area of the ball with a hitting face of a golf club is small.
In order to provide a two-piece solid golf ball having shot feel as good as the thread wound golf ball, a soft type two-piece solid golf ball using a softer core has been proposed. However, the use of the soft core adversely affects on rebound characteristics, thus resulting in the reduction of flight distance and the deterioration of durability.
Then, it has been proposed that an intermediate layer is placed between the core and the cover of the two-piece solid golf ball to form a three-piece solid golf ball so as to keep the balance between flight performance and shot feel at the time of hitting. For example, a three-piece solid golf ball of which a two-piece core composed of a core and an intermediate layer has a specified range of thickness, hardness, specific gravity and the like is suggested. The intermediate layer is formed from vulcanized rubber material having the same composition as the core. For example, Japanese Patent No. 2614791 suggests a three-piece solid golf ball in which an inner core has a diameter of 23 to 35 mm and a Shore D hardness of 30 to 62, an outer core has a diameter of 36 to 41 mm and a Shore D hardness of 30 to 56.
A conventional method of making a solid golf ball containing a two-layered core, including the above technique, generally comprises the steps of vulcanizing a rubber composition for an inner core by press-molding or injection molding to form a vulcanized spherical inner core, covering the inner core with a rubber composition for an outer core, and vulcanizing it again to form a multi-layered core. The producing method of the two-layered core, as explained above, is more complicated than the production of a one-layer core. In addition, the inner core is vulcanized twice including the vulcanization of the outer core and therefore is thermally deteriorated by the twice vulcanization to degrade rebound characteristics and durability.
A main object of the present invention is to provide a multi-piece solid golf ball having excellent durability and excellent rebound characteristics, while keeping good shot feel at the time of hitting.
According to the present invention, the object described above has been accomplished by adjusting a diameter and a center hardness of the inner core, a thickness and a surface hardness of the outer core, and a hardness of the cover to a specified range. The invention uses a method of making the two-layered core comprising the steps of covering an unvulcanized inner core with two vulcanized semi-spherical half-shells for the outer core to integrally mold and vulcanize it, thereby simplifying the manufacturing process. The present invention can provide a multi-piece solid golf ball having excellent durability and excellent rebound characteristics, while keeping good shot feel at the time of hitting.